Conventional running shoes use a mid sole composed of a shock absorbing material that provides extensive cushioning to disperse and alleviate the impact during landing.
However during running, the force applied to the heel is said to be at least three times the bodyweight of the runner. Conventional running shoes have not been able to efficiently convert the impact during landing into energy during the kick phase. Designers have therefore faced the technical problem of how to store the impact during landing as energy and release such energy to the forefoot during the kick phase to make running and walking more efficient.
On the other hand, Japanese Patent 3,377,211 discloses a sports shoe where a hollow is formed at the heel and coil springs are disposed inside the hollow. This sports shoe can absorb the impact and the energy of the impact used to supplement a force that lifts the foot.
Japanese Utility Model 3,070,522 discloses a sports shoe that has a pedal at the toe end of the shoe main body and has a plate spring and another spring provided below the pedal. With this sport shoe, the runner is capable of a high spring due to the repulsive force of the springs.
Also, Japanese Laid-Open Patent Publication No. H10-262706 discloses a shoe that has a plate spring and a coil spring provided at a heel part inside the shoe. According to this shoe, it is possible to use the force of the springs to supplement a force that lifts the heel.
In addition, Japanese Laid-Open Patent Publication No. 2001-112505 discloses a construction for attaching a plate spring to the toe part of a shoe. According to this construction, energy stored by compression of the spring is used to supplement a force that lifts the foot.